JP6551632B1 - Low alloy high strength seamless steel pipe for oil well - Google Patents

Abstract

The number of non-metallic inclusions in the oxide-based steel containing CaO, Al 2 O 3 and MgO having a specific alloy composition and satisfying the following formulas (1) and (2) and having a major axis of 5 μm or more per 100 mm 2 The number of non-metallic inclusions in the oxide-based steel containing CaO, Al 2 O 3 and MgO having a major axis of 5 μm or more satisfying the following formulas (3) and (4) with a composition ratio of 5 or less is 20 per 100 mm 2. A low-alloy high-strength seamless steel pipe for oil wells having excellent SSC resistance in a higher hydrogen sulfide gas saturation environment while having a high strength of 862 MPa or more, which is the following. (CaO) / (Al2O3) ≦ 0.25 (1) 1.0 ≦ (Al2O3) / (MgO) ≦ 9.0 (2) (CaO) / (Al2O3) ≧ 2.33 (3) (CaO) / (MgO) ≧ 1.0 (4) Here, (CaO), (Al 2 O 3), and (MgO) are the mass% of CaO, Al 2 O 3, and MgO in the non-metallic inclusions in the oxide steel, respectively.

Description

  The present invention is a high-strength seamless steel pipe for oil wells and gas wells (hereinafter, also simply referred to as oil wells), and particularly an oil well excellent in sulfide stress corrosion cracking (SSC) in a sour environment containing hydrogen sulfide. This invention relates to low alloy high strength seamless steel pipe. Here, “high strength” refers to the case where the yield strength is 862 MPa or more (125 ksi or more).

  In recent years, from the viewpoint of soaring crude oil prices and the depletion of oil resources expected in the near future, the sour environment is so severe that it includes deep oil fields and hydrogen sulfide that were not previously excluded. Development of oil fields and gas fields, etc. in corrosive environment has become popular. A steel pipe for oil wells used under such an environment is required to have a material that has high strength and excellent corrosion resistance (sourcing resistance).

To such a demand, for example, in Patent Document 1, C: 0.15 to 0.30%, Si: 0.05 to 0.5%, Mn: 0.05 to 1% by weight%. Al: 0.005-0.5%, Cr: 0.2-1.5%, Mo: 0.1-1%, V: 0.05-0.3%, and Nb: 0.003-0 The balance contains Fe and unavoidable impurities, and as impurities, P is not more than 0.025%, S is not more than 0.01%, N is not more than 0.01%, O (oxygen) is not more than 0. It is made of a low alloy of not more than 01%, the total amount of precipitated carbides is 1.5 to 4% by mass, the proportion of MC carbides in the total amount of carbides is 5 to 45% by mass, M ₂₃ C ₆ carbides An oil well with excellent toughness and resistance to sulfide stress corrosion cracking by setting the product thickness to t (mm) (200 / t) or less by mass. Steel for use is disclosed.

Further, in Patent Document 2, C: 0.22 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less by mass%. S: 0.01% or less, Cr: 0.1 to 1.08%, Mo: 0.1 to 1%, Al: 0.005 to 0.1%, B: 0.0001 to 0.01% N: 0.005% or less, O (oxygen): 0.01% or less, Ni: 0.1% or less, Ti: 0.001 to 0.03%, and 0.00008 / N% or less V: 0 to 0.5%, Zr: 0 to 0.1%, Ca: 0 to 0.01%, the balance contains Fe and impurities, and the number of TiN having a diameter of 5 μm or more per 1 mm ² cross section By setting the number to 10 or less, the yield strength is 758 to 862 MPa and the crack initiation limit stress (σth) is 85% or more of the standard minimum strength (SMYS) of the steel material. Steel pipe excellent in sulfide stress corrosion cracking resistance is disclosed.

  On the other hand, in Patent Document 3, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, P: 0.025 by mass%. %: S: 0.01% or less, Al: 0.005-0.10%, Cr: 0.1-1.0%, Mo: 0.5-1.0%, Ti: 0.002- [211] of steel containing 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less, O: 0.01% or less A steel for low-alloy oil country tubular goods having a yield strength of 861 MPa or more, which is excellent in sulfide stress corrosion cracking resistance, is disclosed by prescribing a formula consisting of a half-value width and a hydrogen diffusion coefficient to a predetermined value.

JP, 2000-297344, A JP 2001-131698 A JP 2005-350754 A

  The sulfide stress corrosion cracking resistance of the steel of the technology disclosed in these Patent Documents 1 to 3 refers to a round bar tensile test specimen defined in NACE (abbreviation of National Association of Corrosion Engineering) TM0177 method A. It means the presence or absence of SSC when immersed for 720 hours in a test bath saturated with hydrogen sulfide gas under constant stress.

  Here, with respect to Patent Document 1, evaluation of the SSC test is performed using a 0.5% acetic acid + 5% saline aqueous solution at 25 ° C. saturated with hydrogen sulfide at 0.05 atm (= 0.005 MPa) as a test bath. Is going. Moreover, about patent document 2, the partial pressure of hydrogen sulfide is made into 1 atm (= 0.1 MPa) of C110, and C125-C140 is 1 at 1 atmospheric pressure by making a test bath into 25 degreeC 0.5% acetic acid + 5% salt solution. Since the test is severe, the SSC test is evaluated at 0.1 atm (= 0.01 MPa). Furthermore, about patent document 3, 5 mass% salt + 0.5 mass% acetic acid aqueous solution of normal temperature which saturated the hydrogen sulfide gas (carbonic acid gas balance) of 0.1 atm (= 0.01MPa) as a test bath Hereinafter referred to as "bath A") and 5 mass% of sodium chloride + 0.5 mass% of acetic acid aqueous solution (hereinafter referred to as "B bath") saturated with hydrogen sulfide gas (carbon dioxide gas balance) of 1 atm (= 0.1 MPa) The evaluation of the SSC test is carried out using In particular, in the example of Table 4 of Patent Document 3, all steels having a yield strength of 944 MPa or more are evaluated for the SSC test in the “A bath”. As described above, particularly in the case of steel having a yield strength of 862 MPa or more, the test at a partial pressure of hydrogen sulfide gas of 1 atm (= 0.1 MPa) is severe, and 0.05 atm (= 0.005 MPa) or 0.1 The aim was to pass the SSC test in a test bath saturated with hydrogen sulfide gas at atmospheric pressure (= 0.01 MPa). However, the hydrogen sulfide environment for oil wells and gas wells in recent years has become harsh, and the sulfide resistance stress of high-strength steel pipes for oil wells in an environment saturated with more severe 0.2 atm (= 0.02 MPa) hydrogen sulfide gas. Corrosion cracking resistance is required, and none of the above prior art is sufficient.

  The present invention has been made in view of such problems, and has a high yield strength of 862 MPa or more and a higher hydrogen sulfide gas saturation environment, specifically, a hydrogen sulfide gas partial pressure of 0.02 MPa. An object of the present invention is to provide a low alloy high strength seamless steel pipe for oil wells having excellent sulfide stress corrosion cracking resistance (SSC resistance) under the following sour environment.

In order to solve the above-mentioned problems, the present inventors first performed an SSC test on a seamless steel pipe having various chemical compositions and a yield strength of 862 MPa or more based on NACE TM0177 method A. Incidentally, 0.1 atm as a test bath (= 0.01 MPa), and 0.2 atm (= 0.02 MPa) 2 kinds saturated with hydrogen sulfide gas for 24 ° C. of 0.5 _wt% CH 3 COOH And a mixed aqueous solution of CH ₃ COONa was used. The pH of the test bath was adjusted to 3.5 at the end of saturation of each hydrogen sulfide gas. The test stress in the SSC test was 90% of the actual yield strength of each steel pipe. Furthermore, the number of SSC tests was three for each steel pipe. FIG. 1 shows a graph in which the average of the three break times of the SSC test performed is arranged by the yield strength of each steel pipe. In FIG. 1, the vertical axis is the average (hr) of the breaking time of each of the three SSC tests, and the horizontal axis is the yield strength YS (MPa) of the steel pipe.

  In FIG. 1, hollow plots (using plots of ◯, Δ, □) indicate SSC test results under a hydrogen sulfide gas saturation condition of 0.01 MPa. Under these test conditions, when the yield strength of the steel was in the range of 863 MPa to 933 MPa, none of the three steel pipes broke at the time of 720 hours during the three tests (each plot of ◯, Δ, □). On the other hand, in FIG. 1, the solid plots (using the plots ●, ▲, and ■) show the results of the SSC test under the hydrogen sulfide gas saturation condition of 0.02 MPa. Under these test conditions, regardless of the yield strength of the steel, three of the three tests did not break at 720 hours (● plot), one or more of the three or three of them broke, and When the average of the three break times is about 400 hours or more and less than 720 hours (▲ plot), and when three of the three breaks together, the average of the three break times is less than about 400 hours (■ plot) I found it to be divided.

  Therefore, the inventors diligently conducted researches on the difference between these SSC test results. As a result, it was found that the occurrence position of SSC was different between those having an average rupture time of 400 hours or more and less than 720 hours (▲ plot) and those having less than 400 hours (■ plot). Specifically, by observing the fracture surface of the fracture test piece, those with an average fracture time of 400 hours or more and less than 720 hours (▲ plot), SSC occurred from the surface of the specimen, and the average fracture time was less than 400 hours. As for the thing (■ plot), SSC was generated from the inside of the test piece.

Based on these results, the inventors conducted further studies and found that differences in the distribution of inclusions in the steel change the behavior of these SSCs. Specifically, from the vicinity of the pipe from which the SSC test piece was taken, an observation sample of a 15 mm × 15 mm inspection surface was taken at a cross section in the longitudinal direction of the steel pipe at the thickness position where the SSC test piece was taken. After that, SEM observation of inclusions in a region of 10 mm × 10 mm with a scanning electron microscope (SEM) and the chemical composition of the inclusions were analyzed with a characteristic X-ray analyzer accompanying the SEM, and the mass% was calculated. . As a result, most of the major axis 5μm or more inclusions _Al 2 _O 3, CaO, an oxide containing MgO, and _Al 2 _O 3, CaO, when plotting the respective mass ratio ternary composition diagram of the MgO It has been found that the oxide composition differs depending on the difference in SSC generation behavior described above.

FIG. 2 shows an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe whose average fracture time is 400 hours or more and less than 720 hours in FIG. As shown in FIG. 2, the number of Al ₂ O ₃ -MgO composite inclusions having a relatively small CaO ratio is very large. On the other hand, FIG. 3 shows an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in the steel pipe whose fracture time average was less than 400 hours in FIG. As shown in FIG. 3, in contrast to FIG. 2, the number of CaO-Al ₂ O ₃ -MgO composite inclusions having a large CaO ratio is very large. Furthermore, FIG. 4 shows an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe in which three tubes did not break in 720 hours in FIG. Show. As shown in FIG. 4, it can be seen that the number of inclusions having a small CaO ratio and inclusions having a large CaO ratio is reduced as compared with FIGS. 2 and 3.

  From the above, the inclusion time composition having a fracture time average of 400 hours or more and less than 720 hours, a large amount of inclusions present in the steel pipe where SSC was generated from the surface of the test piece, and the break time average of less than 400 hours, From the comparison with the number of inclusions in the steel pipe in which SSC did not occur in 720 hours, and the number of inclusions in those inclusion compositions in the steel pipe where SSC did not occur in 720 hours, The upper limit of the number of inclusions

The present invention has been completed based on these findings and comprises the following gist.
[1] In mass%, C: 0.25 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.45 to 0.90%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.9 to 1.5%, Mo: 1 .4 to 2.0%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.0040%, Ca: 0.0010 to 0.0020%, Mg: 0.001% or less, N : 0.005% or less of CaO, Al ₂ having a composition comprising the balance Fe and inevitable impurities, and having a compositional ratio satisfying the following formulas (1) and (2) of 5 μm or more in major axis O _3, the number of oxide-based steel in non-metallic inclusions containing MgO is 100 mm ² per 5 or less, the composition ratio of the following equation (3) and 4) more than the major diameter 5μm satisfying the formula _{CaO, Al} 2 O 3, the number of oxide-based steel in non-metallic inclusions containing MgO is at 100 mm ² per 20 or less, oil wells yield strength is not less than 862MPa Low alloy high strength seamless steel pipe for use.
(CaO) / (Al ₂ O ₃ ) ≦ 0.25 (1)
1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0 (2)
(CaO) / (Al ₂ O ₃ ) ≧ 2.33 (3)
(CaO) / (MgO) ≧ 1.0 (4)
Here, (CaO), (Al ₂ O ₃ ) and (MgO) are mass% of CaO, Al ₂ O ₃ and MgO in non-metallic inclusions in oxide-based steel, respectively.
[2] In addition to the above-mentioned composition, it is further selected by mass% from V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0.3% The low-alloy high-strength seamless steel pipe for oil wells according to the above [1], which contains one or two or more kinds.
[3] In addition to the above composition, the composition further contains one or two kinds selected from Ti: 0.003 to 0.050% and Zr: 0.005 to 0.10% by mass%. The low alloy high strength seamless steel pipe for oil wells according to [1] or [2].

In addition, "high strength" here refers to having the strength whose yield strength is 862 Mpa or more (125 ksi or more).
In the present invention, excellent resistance to sulfide stress corrosion cracking (SSC resistance) is an SSC test based on NACE TM0177 methodA, and in particular, hydrogen sulfide gas at 0.2 atm (= 0.02 MPa) is used. _Three SSC tests each using a saturated aqueous solution of 0.5% by mass CH ₃ COOH and CH ₃ COONa at 24 ° C as test baths, all of which have a break time of 720 hours or more. Point to
In the present invention, the oxide system containing CaO, Al ₂ O ₃ and MgO is formed by the reaction between Ca added for the purpose of shape control of MnS in steel and the like and O contained in molten steel. CaO, and when the molten steel refined by a converter method or the like is taken out into a ladle, or is generated by the reaction of deoxidized material Al added in the molten steel with O contained in the molten steel The reaction between Al ₂ O ₃ and also the refractory of the MgO-C composition of the ladle and the CaO-Al ₂ O ₃ -SiO ₂ -based slag used for desulfurization during the desulfurization treatment of molten steel, the molten steel It refers to what remains in the steel after solidification as oxides such as MgO which has been dissolved in during aggregation and combination during casting such as continuous casting or ingot casting.

  According to the present invention, excellent sulfide stress in a higher hydrogen sulfide gas saturation environment, specifically, a sour environment with a hydrogen sulfide gas partial pressure of 0.02 MPa or less, while having a high yield strength of 862 MPa or more. A low-alloy high-strength seamless steel pipe for oil wells exhibiting corrosion cracking resistance (SSC resistance) can be provided.

FIG. 1 is a graph of the yield strength of a steel pipe and the average breaking time of three SSC tests. FIG. 2 is an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe whose break time average was 400 hours or more and less than 720 hours in the SSC test. FIG. 3 is an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe whose break time average was less than 400 hours in the SSC test. FIG. 4 is an example of a ternary composition diagram of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe in which three triple fractures did not occur during three hours in 720 hours in the SSC test.

  Hereinafter, the present invention will be described in detail.

The low-alloy high-strength seamless steel pipe for oil wells of the present invention is mass%, C: 0.25 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.45 to 0.90%. , P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0 .9 to 1.5%, Mo: 1.4 to 2.0%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.0040%, Ca: 0.0010 to 0.0020 %, Mg: 0.001% or less, N: 0.005% or less, the balance is Fe and inevitable impurities, and the composition has the following composition ratios (1) and (2) The number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO with a major axis of 5 μm or more satisfying the above requirement is 5 per 100 mm ² Hereinafter, the number of nonmetallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major diameter of 5 μm or more satisfying the following formulas (3) and (4): 20 per 100 mm ² The yield strength is 862 MPa or more. Moreover, in addition to the above-mentioned composition, further, by mass%, V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0.3%, it is selected from among It can contain one or more of the following. Furthermore, 1 type or 2 types selected from Ti: 0.003-0.050% and Zr: 0.005-0.10% can be contained by mass%.
(CaO) / (Al ₂ O ₃ ) ≦ 0.25 (1)
1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0 (2)
(CaO) / (Al ₂ O ₃ ) ≧ 2.33 (3)
(CaO) / (MgO) ≧ 1.0 (4)
Here, (CaO), (Al ₂ O ₃ ) and (MgO) are mass% of CaO, Al ₂ O ₃ and MgO in non-metallic inclusions in oxide-based steel, respectively.

  First, the reasons for limitation of the chemical composition of the steel pipe of the present invention will be described. Hereinafter, mass% is simply expressed as% unless otherwise specified.

C: 0.25 to 0.50%
C has an effect of increasing the strength of steel and is an important element for ensuring a desired high strength. In order to achieve a high yield strength of 862 MPa or more, which is the target yield strength in the present invention, it is necessary to contain 0.25% or more of C. On the other hand, the content of C exceeding 0.50% significantly inhibits the resistance to sulfide stress corrosion cracking without decreasing the hardness even after high temperature tempering. Therefore, C is set to 0.25 to 0.50%. C is preferably 0.26% or more, and more preferably 0.27% or more. C is preferably 0.40% or less, more preferably 0.30% or less.

Si: 0.01-0.40%
Si is an element which acts as a deoxidizing agent, is solid-solved in the steel to increase the strength of the steel, and has the function of suppressing rapid softening during tempering. In order to obtain such an effect, it is necessary to contain 0.01% or more of Si. On the other hand, when the content of Si exceeds 0.40%, coarse oxide-based inclusions are formed and become the starting point of SSC. For this reason, Si is made 0.01 to 0.40%. Si is preferably 0.02% or more. Si is preferably 0.15% or less, more preferably 0.04% or less.

Mn: 0.45 to 0.90%
Mn is an element that has the effect of increasing the strength of steel through the improvement of hardenability and binding to S to fix S as MnS to prevent grain boundary embrittlement due to S. In the present invention, it is necessary to contain 0.45% or more of Mn. On the other hand, when the content of Mn exceeds 0.90%, the hardness of the steel is significantly increased, and even if high temperature tempering is performed, the hardness does not decrease and the sulfide stress corrosion cracking susceptibility is significantly impaired. Therefore, Mn is set to 0.45 to 0.90%. Mn is preferably 0.55% or more, more preferably 0.60% or more. Mn is preferably 0.85% or less, more preferably 0.80% or less.

P: 0.010% or less P is segregated in grain boundaries and the like in a solid solution state, and tends to cause intergranular embrittlement cracking and the like. In the present invention, it is desirable to reduce as much as possible, but 0.010% is acceptable. Because of this, P is made 0.010% or less. P is preferably 0.009% or less, more preferably 0.008% or less.

S: 0.001% or less S is mostly present as sulfide inclusions in steel, and reduces the ductility, toughness, and corrosion resistance such as sulfide stress corrosion cracking resistance. A part of S may be present in a solid solution state, but in that case, it tends to segregate at grain boundaries and the like and cause intergranular brittleness cracking and the like. For this reason, it is desirable to reduce S as much as possible in the present invention, but excessive reduction raises the refining cost. For this reason, in the present invention, S is set to 0.001% or less where the adverse effect is acceptable.

O (oxygen): 0.0015% or less O (oxygen) is present as an inevitable impurity in the steel as an oxide of Al, Si, Mg, Ca or the like. As will be described later, in the SSC test, in particular, the composition ratio satisfying (CaO) / (Al ₂ O ₃ ) ≦ 0.25 and 1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0. When the number of oxides having a major axis of 5 μm or more exceeds 5 per 100 mm ² , these oxides are the starting points, and SSC that breaks from the surface of the test piece in a long time occurs. In the SSC test, the number of oxides having a major axis of 5 μm or more with a composition ratio satisfying (CaO) / (Al ₂ O ₃ ) ≧ 2.33 and (CaO) / (MgO) ≧ 1.0 per 100 mm ² When it exceeds 20, these oxides are the starting point, and SSC which breaks in a short time from the inside of the test piece is generated. For this reason, O (oxygen) is made 0.0015% or less to which the adverse effect is allowable. O (oxygen) is preferably 0.0012% or less, more preferably 0.0010% or less.

Al: 0.015 to 0.080%
Al acts as a deoxidizing agent and combines with N to form AlN, which contributes to the reduction of solid solution N. In order to acquire such an effect, Al needs to contain 0.015% or more. On the other hand, if the Al content exceeds 0.080%, the cleanliness in the steel is lowered, and as described later, in the SSC test, in particular, (CaO) / (Al ₂ O ₃ ) ≦ 0.25, and When the number of oxides with a major axis of 5 μm or more having a composition ratio satisfying 1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0 exceeds 5 per 100 mm ² , these oxides serve as starting points. , SSC is generated from the surface of the test piece for a long time. For this reason, Al is made 0.015 to 0.080% whose adverse effect is acceptable. Al is preferably 0.025% or more, more preferably 0.050% or more. Al is preferably 0.075% or less, more preferably 0.070% or less.

Cu: 0.02 to 0.09%
Cu is an element having an action of improving the corrosion resistance. When a small amount of Cu is contained, a dense corrosion product is formed, the formation and growth of pits serving as the starting point of SSC are suppressed, and the resistance to sulfide stress corrosion cracking is significantly improved. For this reason, in the present invention, it is necessary to contain 0.02% or more of Cu. On the other hand, if the content of Cu exceeds 0.09%, the hot workability in the manufacturing process of the seamless steel pipe is reduced. For this reason, Cu is made into 0.02 to 0.09%. Cu is preferably 0.07% or less, more preferably 0.04% or less.

Cr: 0.9 to 1.5%
Cr is an element that contributes to an increase in the strength of the steel and improves the corrosion resistance through an increase in hardenability. In addition, Cr combines with C during tempering to form carbides such as M ₃ C, M ₇ C ₃ and M ₂₃ C ₆ systems. In particular, the M ₃ C-based carbide improves the temper softening resistance, reduces the strength change due to tempering, and contributes to the improvement of the yield strength. In order to achieve the yield strength of 862 MPa or more targeted by the present invention, the inclusion of Cr of 0.9% or more is required. On the other hand, the content of Cr exceeding 1.5% significantly increases the hardness of the steel, and even if high temperature tempering is performed, the hardness does not decrease and the sulfide stress corrosion cracking susceptibility is significantly impaired. For this reason, Cr is made into 0.9 to 1.5%. Cr is preferably 1.0% or more. Cr is preferably 1.3% or less.

Mo: 1.4 to 2.0%
Mo is an element that contributes to an increase in the strength of steel through the increase in hardenability and improves the corrosion resistance. In particular, Mo ₂ C carbides secondarily precipitated after tempering improve temper softening resistance, reduce strength change due to tempering, and contribute to improvement of yield strength. In addition, in the steel having a yield strength of 862 MPa or more as intended in the present invention, by adding a specific amount of Mo, particularly in a sour environment of hydrogen sulfide gas partial pressure of 0.2 atm (0.02 MPa) or more, Crack propagation resistance of sulfide stress corrosion cracking is improved, and high yield strength and sulfide stress corrosion cracking resistance are compatible. In order to obtain such an effect, it is necessary to contain 1.4% or more of Mo. On the other hand, when the content of Mo exceeds 2.0%, Mo ₂ C carbides become coarse, and as a starting point of sulfide stress corrosion cracking, SSC is generated. For this reason, Mo is made into 1.4 to 2.0%. Mo is preferably 1.5% or more. Mo is preferably 1.8% or less.

Nb: 0.005 to 0.05%
Nb retards recrystallization in the austenite (γ) temperature region, contributes to the refinement of γ grains, and works extremely effectively for the refinement of the steel substructure (eg, packet, block, lath) immediately after quenching. Element. In order to obtain such an effect, it is necessary to contain Nb of 0.005% or more. On the other hand, if Nb content exceeds 0.05%, the hardness of the steel is remarkably increased, and even if high temperature tempering is performed, the hardness does not decrease and the resistance to sulfide stress corrosion cracking is significantly inhibited. Therefore, Nb is set to 0.005 to 0.05%. Nb is preferably 0.006% or more, and more preferably 0.007% or more. Nb is preferably 0.030% or less, and more preferably 0.010% or less.

B: 0.0005 to 0.0040%
B is an element which contributes to the improvement of the hardenability with a slight content. In this invention, 0.0005% or more of B needs to be contained. On the other hand, even if B is contained in excess of 0.0040%, the above effect is saturated or the formation of Fe boride (Fe-B) makes it impossible to expect the desired effect, which is economically disadvantageous. It becomes. Therefore, B is set to 0.0005 to 0.0040%. B is preferably 0.0010% or more, and more preferably 0.0015% or more. B is preferably 0.0030% or less, and more preferably 0.0025% or less.

Ca: 0.0010 to 0.0020%
Ca is positively added to control the form of oxide inclusions in the steel. As described above, in the SSC test, the number of composite oxides mainly composed of Al ₂ O ₃ —MgO having an (Al ₂ O ₃ ) / (MgO) ratio of 1.0 to 9.0 is 5 per 100 mm ^2. If more than one are present, these oxides form SSCs that break from the surface of the test piece over a long period of time. In order to suppress the formation of such a composite oxide mainly composed of Al ₂ O ₃ —MgO, the present invention needs to contain 0.0010% or more of Ca. On the other hand, in the SSC test, the content of Ca exceeding 0.0020% has a composition ratio that satisfies (CaO) / (Al ₂ O ₃ ) ≧ 2.33 and (CaO) / (MgO) ≧ 1.0. This causes an increase in the number of oxides having a major axis of 5 μm or more, and these oxides form SSCs that break within a short time from the inside of the test piece. Therefore, Ca is set to 0.0010 to 0.0020%. Ca is preferably 0.0012% or more. Ca is preferably 0.0017% or less.

Mg: not more than 0.001% Mg is not added positively, but during desulfurization treatment such as Ladle furnace (LF) performed for low S, refractory of MgO-C composition of ladle, It enters into molten steel as an Mg component by reaction with CaO—Al ₂ O ₃ —SiO ₂ slag used for desulfurization. As described above, in the SSC test, the number of composite oxides mainly composed of Al ₂ O ₃ —MgO having an (Al ₂ O ₃ ) / (MgO) ratio of 1.0 to 9.0 is 5 per 100 mm ^2. If more than one are present, these oxides form SSCs that break from the surface of the test piece over a long period of time. For this reason, Mg is made 0.001% or less at which the adverse effect is acceptable. Mg is preferably 0.0008% or less, more preferably 0.0005% or less.

N: 0.005% or less N is an unavoidable impurity in steel and combines with a nitride-forming element such as Ti, Nb or Al to form a MN-type precipitate. Furthermore, the remaining surplus N forming these nitrides combines with B to also form BN precipitates. At this time, since the effect of improving hardenability due to the addition of B is lost, it is desirable to reduce surplus N as much as possible. For this reason, N is made 0.005% or less. N is preferably 0.004% or less.

  The balance other than the above components is Fe and unavoidable impurities.

  In the present invention, in addition to the above basic composition, V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0. One or more selected from 3% can be contained. Furthermore, it can contain 1 type or 2 types chosen from Ti: 0.003-0.050% and Zr: 0.005-0.10% by the mass%.

V: 0.02-0.3%
V is an element which forms carbides or nitrides and contributes to strengthening of the steel. In order to acquire such an effect, it is preferable to make it contain V 0.02% or more. On the other hand, when V is contained exceeding 0.3%, the V-based carbide becomes coarse and becomes a starting point of sulfide stress corrosion cracking, which may cause SSC. For this reason, when V is contained, V is preferably 0.02 to 0.3%. V is more preferably 0.03% or more. More preferably, it is 0.04% or more. V is more preferably 0.1% or less. More preferably, it is 0.06% or less.

W: 0.03-0.2%
W is also an element that forms carbides or nitrides and contributes to strengthening of the steel. In order to acquire such an effect, it is preferable to make it contain W 0.03% or more. On the other hand, when W is contained in excess of 0.2%, the W-based carbide becomes coarse and becomes a starting point of sulfide stress corrosion cracking, which may cause SSC. For this reason, when it contains W, it is preferable to make W into 0.03 to 0.2%. W is more preferably 0.07% or more, and more preferably 0.1% or less.

Ta: 0.03-0.3%
Ta is also an element that forms carbides or nitrides and contributes to strengthening of steel. In order to obtain such an effect, it is preferable to contain 0.03% or more of Ta. On the other hand, if the content of Ta exceeds 0.3%, the Ta-based carbides coarsen and become a starting point of sulfide stress corrosion cracking, and there is a possibility that SSC may be generated. For this reason, when it contains Ta, it is preferable to make Ta into 0.03 to 0.3%. Ta is more preferably 0.08% or more, and more preferably 0.2% or less.

Ti: 0.003 to 0.050%
Ti is an element that forms nitrides and contributes to prevention of coarsening due to the pinning effect of austenite grains during steel quenching. Furthermore, the resistance to hydrogen sulfide cracking is improved by making the austenite grains fine. In particular, the required austenite grain refinement can be achieved without repeating the quenching (Q) and tempering (T) described later twice or three times. In order to obtain such an effect, it is preferable to contain 0.003% or more of Ti. On the other hand, if the Ti content exceeds 0.050%, the coarse Ti-based nitride becomes the starting point of sulfide stress corrosion cracking, and SSC may occur. For this reason, when it contains Ti, it is preferable to make Ti into 0.003 to 0.050%. Ti is more preferably 0.005% or more. More preferably, it is 0.010% or more. Ti is more preferably 0.025% or less. More preferably, it is 0.018% or less.

Zr: 0.005 to 0.10%
Zr also forms nitrides like Ti, prevents coarsening due to the pinning effect of austenite grains during quenching of steel, and improves resistance to hydrogen sulfide cracking. In particular, the effect is remarkable by complex addition with Ti. In order to obtain such an effect, it is preferable to contain 0.005% or more of Zr. On the other hand, when the content of Zr exceeds 0.10%, the coarsened Zr-based nitride or Ti-Zr composite nitride becomes a starting point of sulfide stress corrosion cracking, and there is a possibility that SSC occurs. For this reason, when it contains Zr, it is preferable to make Zr into 0.005 to 0.10%. The Zr content is more preferably 0.013% or more, and more preferably 0.026% or less.

  Next, as the structure of the steel pipe of the present invention, the definition of inclusions in steel will be described.

The number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major axis of 5 μm or more and a composition ratio satisfying the following equations (1) and (2) is 5 or less per 100 mm ² CaO) / (Al ₂ O ₃ ) ≦ 0.25 (1)
1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0 (2)
Here, (CaO), (Al ₂ O ₃ ) and (MgO) are mass% of CaO, Al ₂ O ₃ and MgO in non-metallic inclusions in oxide-based steel, respectively.

As described above, it is a mixed aqueous solution of 0.5 mass% CH ₃ COOH and CH ₃ COONa at 24 ° C. saturated with 0.02 MPa of hydrogen sulfide gas, and the pH is 3. In the test bath adjusted to be 5, the test stress was 90% of the actual yield strength of the steel pipe, and three SSC tests were performed for each steel pipe. As shown in FIG. 2, the ternary composition of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of 5 μm or more in a steel pipe having an average breaking time of 400 hours or more and less than 720 hours in the SSC test. In the (CaO) / (Al ₂ O ₃ ) ratio, the proportion of Al ₂ O ₃ is large, and in the (Al ₂ O ₃ ) / (MgO) ratio, a large proportion of Al ₂ O ₃ is present. . Expressions (1) and (2) show this range quantitatively. Furthermore, in the SSC test, when all the test pieces were not broken in 720 hours, the number of inclusions of 5 μm or more in the same inclusion composition of the steel pipe was 720 if the number was 5 or less per 100 mm ^2. It was found that it did not break in time. Therefore, the number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major diameter of 5 μm or more satisfying the equations (1) and (2) is 5 or less per 100 mm ^2. . Preferably, it is 3 or less. In addition, inclusions of these compositions were exposed on the surface of the test piece as the reason that inclusions having a major diameter of 5 μm or more satisfying such expressions (1) and (2) adversely affect sulfide stress corrosion cracking resistance. In this case, first, the inclusions themselves dissolve in the test bath, and then pitting progresses gradually, and it is considered that SSC is generated from the pitting portion at about 400 hours and leads to breakage.

The number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major axis of 5 μm or more and a composition ratio satisfying the following equations (3) and (4) is 20 or less per 100 mm ² ( CaO) / (Al ₂ O ₃ ) ≧ 2.33 (3)
(CaO) / (MgO) ≧ 1.0 (4)
Here, (CaO), (Al ₂ O ₃ ) and (MgO) are mass% of CaO, Al ₂ O ₃ and MgO in non-metallic inclusions in oxide-based steel, respectively.

As described above, it is a mixed aqueous solution of 0.5 mass% CH ₃ COOH and CH ₃ COONa at 24 ° C. saturated with 0.02 MPa of hydrogen sulfide gas, and the pH is 3. In the test bath adjusted to be 5, the test stress was 90% of the actual yield strength of the steel pipe, and three SSC tests were performed for each steel pipe. In this SSC test, the ternary composition of Al ₂ O ₃ , CaO, and MgO of inclusions having a major axis of a steel pipe having a fracture diameter of less than 400 hours and an average fracture time of less than 400 hours, as shown in FIG. In the / (Al ₂ O ₃ ) ratio, there are many cases in which the ratio of CaO is large and in the ratio of (CaO) / (MgO), the ratio of CaO is large. Equations (3) and (4) show this range quantitatively. Furthermore, in the SSC test, when all the test pieces were not broken in 720 hours, the number of inclusions of 5 μm or more in the same inclusion composition of the steel pipe was 720 if the number was 20 or less per 100 mm ^2. It was found that it did not break in time. Therefore, the number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major diameter of 5 μm or more satisfying the equations (3) and (4) is 20 or less per 100 mm ^2. . Preferably, it is 10 or less. As the reason why inclusions with a major diameter of 5 μm or more that satisfy such expressions (3) and (4) adversely affect sulfide stress corrosion cracking resistance, CaO is (CaO) / (Al ₂ O ₃ ) ratio The higher the proportion, the higher the crystallization temperature of inclusions in the molten steel, and as a result, the inclusion size becomes very coarse. At the time of the SSC test, these coarse inclusions and the gap between the ground iron interface are the starting points, and it is considered that SSC is rapidly generated from the inside of the test piece, leading to breakage.

  Next, a method of producing a low alloy high strength seamless steel pipe for oil well, which is excellent in sulfide stress corrosion cracking resistance (SSC resistance) will be described.

  In the present invention, the method for producing a steel pipe material having the above-mentioned composition is not particularly limited. For example, molten steel having the above composition is melted by a generally known melting method such as a converter, electric furnace, vacuum melting furnace, etc., and billet is obtained by a conventional method such as a continuous casting method or an ingot-bundling rolling method. And other steel pipe materials.

In particular, in order to make the number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major axis of 5 μm or more having the two types of inclusion compositions described above, the converter, It is preferable to carry out deoxidation treatment with Al immediately after melting by a generally known melting method such as an electric furnace or a vacuum melting furnace. Furthermore, desulfurization treatment such as ladle furnace (LF) is continued to reduce sulfur (S) in the molten steel, and then N and O (oxygen) in the molten steel are reduced by the degassing apparatus, and then Ca addition is performed. It is preferred to carry out the treatment and to cast it last. Furthermore, after the end of the degassing treatment, the concentration of Ca in the alloy raw material used during the degassing treatment is reduced as much as possible so that the Ca concentration in the molten steel before carrying out the Ca addition treatment becomes 0.0004 mass% or less. It is preferable to implement management.
When the Ca concentration in the molten steel before carrying out the Ca addition treatment exceeds 0.0004 mass%, in the molten steel instead of the case where it is added at an appropriate Ca addition amount [% Ca *] when performing the Ca addition treatment described later. As a result of the increase in Ca concentration, the number of CaO—Al ₂ O ₃ —MgO composite oxides in which the CaO ratio is high and the (CaO) / (MgO) ratio is 1.0 or more increases. As a result, in the SSC test, these oxides are the starting point, and the SSC which is broken in a short time from the inside of the test piece is generated. When the Ca addition treatment is performed after the degassing treatment, oxygen in molten steel [% T.H. It is preferable to add so that it may become appropriate Ca concentration (ratio with respect to molten steel weight, [% Ca *]) according to O value. For example, according to the following equation (5), oxygen in molten steel [% T. The appropriate Ca concentration [% Ca *] can be determined according to the O] value.
0.63 ≦ [% Ca *] / [% T.P. O] ≦ 0.91 (5)
Here, [% Ca *] / [% T. When O] is less than 0.63, as a result of insufficient Ca addition, the CaO ratio is low and the (Al ₂ O ₃ ) / (MgO) ratio is 1. even if the Ca value of the steel pipe is within the range of the present invention. The number of composite oxides mainly composed of Al ₂ O ₃ —MgO that increases from 0 to 9.0 increases. As a result, in the SSC test, these oxides are the starting point, and SSC which is broken in a long time from the surface of the test piece is generated. Meanwhile, [% Ca *] / [% T. When O] exceeds 0.91, the number of CaO—Al ₂ O ₃ —MgO composite oxides having a high CaO ratio and a (CaO) / (MgO) ratio of 1.0 or more increases. As a result, in the SSC test, these oxides are the starting point, and the SSC which is broken in a short time from the inside of the test piece is generated.

The obtained steel pipe material is formed into a seamless steel pipe by hot forming. The hot forming method can be carried out by a generally known method. For example, as a hot forming method, a steel pipe material is heated, pierced by piercing, then formed into a predetermined thickness using a method of mandrel mill rolling or plug mill rolling, and then hot rolling up to appropriate diameter reduction rolling Is called. Here, the heating temperature of the steel pipe material is preferably in the range of 1150 to 1280 ° C. When the heating temperature is less than 1150 ° C., the deformation resistance of the steel pipe material at the time of heating is large, and the piercing failure is caused. On the other hand, when the heating temperature exceeds 1280 ° C., the microstructure becomes extremely coarse, and it becomes difficult to make fine particles during quenching, which will be described later. The heating temperature is preferably 1150 ° C. or higher, and preferably 1280 ° C. or lower. The heating temperature is more preferably 1200 ° C. or higher.
Moreover, it is preferable to make rolling completion temperature into the range of 750-1100 degreeC. When the rolling end temperature is less than 750 ° C., the load applied during the diameter reduction rolling is large, resulting in poor molding. On the other hand, if the rolling finish temperature exceeds 1100 ° C., grain refining by rolling recrystallization is insufficient, and grain refining at the time of quenching described later becomes difficult. The rolling end temperature is preferably 900 ° C. or higher, and preferably 1080 ° C. or lower.
In the present invention, it is preferable to carry out direct quenching (DQ) after hot rolling from the viewpoint of grain refining.

After the seamless steel pipe is formed, the steel pipe is quenched (Q) and the steel pipe is tempered (T) in order to achieve the target yield strength of 862 MPa or more in the present invention. The quenching temperature at this time is preferably 930 ° C. or less from the viewpoint of finer graining. On the other hand, when the quenching temperature is less than 860 ° C., solid solution of secondary precipitation strengthening elements such as Mo or V, W, and Ta is insufficient, and secondary precipitation can not be secured at the end of the subsequent tempering. Therefore, the quenching temperature is preferably set to 860 to 930 ° C.
The tempering temperature needs to be below Ac ₁ temperature to avoid austenite re-transformation, but if it is less than 600 ° C., secondary precipitation of carbide of Mo or V, W, or Ta can not be secured. Therefore, the tempering temperature is preferably at least 600 ° C. or higher. In particular, the final tempering temperature is preferably 630 ° C. or more, more preferably 650 ° C. or more.
Furthermore, it is preferable to repeat the quenching (Q) and the tempering (T) at least twice or more in order to improve the resistance to hydrogen sulfide cracking by grain refinement. When Ti and Zr are not added, it is preferable to repeat three times or more.
When DQ cannot be applied after hot rolling, Ti and Zr are added together, or at least three times or more are quenched (Q) and tempered (T), and the initial quenching temperature is 950 ° C. or more. It is preferable to substitute the effect of DQ.

  Hereinafter, the present invention will be further described in detail based on examples. The present invention is not limited to the following examples.

Example 1
After melting steel with the composition shown in Table 1 by the converter method, Al deoxidation was performed immediately, followed by secondary refining in the order of LF-degassing, followed by Ca addition, and finally continuous. Casting was carried out to produce a steel pipe material. Here, except for a part, Al deoxidizing, LF, and an alloy raw material used at the time of degassing treatment used a high purity one containing no Ca impurity. Then, after degassing treatment, a molten steel sample was collected and analyzed for Ca in molten steel. The analysis results are shown in Table 2-1 and Table 2-2. In addition, oxygen in molten steel [% T.O. O] For [% Ca *] which is a ratio of analyzed value and Ca addition amount to molten steel weight, [% Ca *] / [% T. O] value was calculated and described in Table 2-1 and Table 2-2.

  About continuous casting, it carried out about two types, round billet continuous casting whose slab cross-sectional shape is a circle, and Bloom continuous casting whose same shape is a rectangle. Further, the bloom continuous cast slab was heated and held at about 1200 ° C., and then rolled into a round billet. In addition, in Table 2-1 and Table 2-2, round billet continuous casting was described as "directly cast billet", steel strip rolling was performed, and what was formed into a round billet was described as "steel piece rolled billet". Next, using these round billet materials, hot rolling of seamless steel pipes was performed at billet heating temperatures and rolling finish temperatures shown in Table 2-1 and Table 2-2. Next, heat treatment was performed on these seamless steel pipes at the quenching (Q) temperature and the tempering (T) temperature described in Table 2-1 and Table 2-2. In addition, direct hardening (DQ) was implemented about a part of seamless steel pipe, and heat treatment was performed about the other seamless steel pipes after air cooling.

  At the end of final tempering, a 15 mm × 15 mm inspection surface inclusion inspection sample, a tensile test piece and an SSC test piece were respectively collected from any one thickness center in the circumferential direction of the pipe end. In particular, three SSC specimens were collected. And it evaluated by the following methods.

  After the mirror polishing, the inclusion investigation sample was observed with a scanning electron microscope (SEM) for SEM observation of inclusions in a 10 mm × 10 mm region, and the chemical composition of the inclusions was measured with a characteristic X-ray analyzer attached to the SEM. It analyzed and calculated the mass%. And the number of inclusions having a major diameter of 5 μm or more satisfying the composition ratio of the equations (1) and (2), and the equations (3) and (4), respectively, is counted and listed in Tables 2-1 and 2-2. .

  Next, the tensile test of JIS Z2241 was performed using the collected tensile test piece, and the yield strength was measured. The yield strength of the steel pipe obtained by the test is shown in Table 2-1 and Table 2-2. Here, the yield strength passed 862 MPa or more.

Furthermore, the SSC test was performed based on NACE TM0177 method A using the collected SSC test piece. As a test bath, a mixed aqueous solution of 0.5 mass% CH ₃ COOH and CH ₃ COONa at 24 ° C. saturated with 0.2 atm (= 0.02 MPa) hydrogen sulfide gas was used. The pH of the test bath was adjusted to 3.5 at the end of saturation of each hydrogen sulfide gas. The test stress in the SSC test was 90% of the actual yield strength of each steel pipe. The test time was 720 hours, but those that were not broken at 720 hours were broken or continued until 900 hours were reached. Tables 2-1 and 2-2 show the break times of the three SSC test pieces obtained in the test, respectively. Here, in the SSC test, all three test pieces having a break time of 720 hours or more were regarded as pass.

  The chemical composition, the number of inclusions with a major axis of 5 μm or more with a composition satisfying the formulas (1) and (2), and the number of inclusions with a major axis of 5 μm or more with a composition satisfying the expressions (3) and (4) are all Inventive examples (steel pipe No. 1-1 and steel pipe Nos. 1-6 to 1-14), which were within the scope of the invention, all had a yield strength of 862 MPa or more, and three break times of three SSC tests were conducted. Both were over 720 hours.

  On the other hand, a comparative example in which Ca of the chemical composition exceeds the scope of the present invention (Steel pipe No. 1-2), and the concentration of Ca in molten steel after degassing treatment is high, and [% Ca *] / % T. As a result of the O] value exceeding 0.91, a comparative example (steel pipe No. 1-) in which the number of inclusions having a major axis of 5 μm or more with a composition ratio satisfying the formulas (3) and (4) was outside the scope of the present invention. 3) 2 or more out of 3 SSC test fractured within 720 hours.

  In addition, [% Ca *] / [% T. As a result of the O] value being less than 0.63, the number of inclusions having a major axis of 5 μm or more with a composition ratio satisfying the formulas (1) and (2) was outside the range of the present invention (steel pipe No. 1-4 ) And a comparative example (steel pipe No. 1-5) in which Ca was below the range of the present invention, one or more of the three SSC tests broke within 720 hours.

  Comparative examples (steel pipe Nos. 1-15, 1-17, 1-23, 1-25, 1-27) in which the chemical compositions C, Mn, Cr, Mo, and Nb exceeded the scope of the present invention were tempered at high temperature. Since the strength was still high even after the test, the three SSC tests were broken within 720 hours.

  Conversely, the comparative examples (steel pipe Nos. 1-16, 1-18, 1-24, 1-28) in which the chemical compositions C, Mn, Cr, and B are below the scope of the present invention have the target yield strength. It did not achieve.

  Moreover, the comparative example (steel pipe No. 1-26) in which Mo was less than the range of the present invention was insufficient in the sulfide stress corrosion cracking propagation resistance of steel, and the three SSC tests broke within 720 hours.

  In the comparative examples (steel pipe Nos. 1-19 and 1-20) in which the chemical compositions P and S exceeded the scope of the present invention, two or more of the three SSC tests broke within 720 hours.

  O (oxygen) in the chemical composition exceeds the range of the present invention, and the number of inclusions with a major axis of 5 μm or more satisfying the formulas (1) and (2), and the formulas (3) and (4) are satisfied. The comparative example (steel pipe No. 1-21) in which the number of inclusions having a major axis of 5 μm or more in the composition ratio was out of the scope of the present invention broke within 720 hours for all three SSC tests.

  In the comparative example (steel pipe No. 1-22) in which the Al in the chemical composition exceeded the range of the present invention, the number of inclusions having a major axis of 5 μm or more with a composition ratio satisfying the formulas (1) and (2) was also outside the range of the present invention. The three SSC tests were broken within 720 hours.

  Comparative example (steel pipe No. 1-29) in which Mg having a chemical composition exceeded the range of the present invention and the number of inclusions having a major axis of 5 μm or more satisfying the formulas (1) and (2) was out of the range of the present invention. 2) 2 of 3 SSC tests were broken within 720 hours.

  In the comparative example (steel pipe No. 1-30) in which the chemical composition N exceeded the scope of the present invention, the excess N was combined with B to form BN, so that the solid solution B was insufficient and the hardenability was reduced. The target yield strength was not achieved.

Example 2
After melting steel with the composition shown in Table 3 by the converter method, Al deoxidation was performed immediately, followed by secondary refining in the order of LF-degassing treatment, followed by Ca addition treatment, and finally continuous Casting was carried out to produce a steel pipe material. Here, except for a part, Al deoxidizing, LF, and an alloy raw material used at the time of degassing treatment used a high purity one containing no Ca impurity. Then, after degassing treatment, a molten steel sample was collected and analyzed for Ca in molten steel. The analysis results are shown in Tables 4-1 and 4-2. In addition, oxygen in molten steel [% T.O. O] For [% Ca *] which is a ratio of analyzed value and Ca addition amount to molten steel weight, [% Ca *] / [% T. O] value was calculated and described in Table 4-1 and Table 4-2.

  For continuous casting, round billet continuous casting in which the slab cross-sectional shape is circular was performed. Next, using these round billet materials, hot rolling of seamless steel pipes was performed at billet heating temperatures and rolling finish temperatures shown in Tables 4-1 and 4-2. Next, heat treatment was performed on these seamless steel pipes at the quenching (Q) temperature and the tempering (T) temperature described in Table 4-1 and Table 4-2. In addition, direct hardening (DQ) was implemented about a part of seamless steel pipe, and heat treatment was performed about the other seamless steel pipes after air cooling.

  At the end of final tempering, a 15 mm × 15 mm inspection surface inclusion inspection sample, a tensile test piece and an SSC test piece were respectively collected from any one thickness center in the circumferential direction of the pipe end. In particular, three SSC specimens were collected. And it evaluated by the following methods.

  After the mirror polishing, the inclusion investigation sample was observed with a scanning electron microscope (SEM) for SEM observation of inclusions in a 10 mm × 10 mm region, and the chemical composition of the inclusions was measured with a characteristic X-ray analyzer attached to the SEM. It analyzed and calculated the mass%. And the number of inclusions having a major diameter of 5 μm or more satisfying the composition ratio of the equations (1) and (2), and the equations (3) and (4) is counted, and the results are listed in Table 4-1 and Table 4-2. .

  Next, the tensile test of JIS Z2241 was performed using the collected tensile test piece, and the yield strength was measured. The yield strength of the steel pipe obtained in the test is shown in Table 4-1 and Table 4-2. Here, the yield strength passed 862 MPa or more.

Furthermore, the SSC test was performed based on NACE TM0177 method A using the collected SSC test piece. As a test bath, a mixed aqueous solution of 0.5 mass% CH ₃ COOH and CH ₃ COONa at 24 ° C. saturated with 0.2 atm (= 0.02 MPa) hydrogen sulfide gas was used. The pH of the test bath was adjusted to 3.5 at the end of saturation of each hydrogen sulfide gas. The test stress in the SSC test was 90% of the actual yield strength of each steel pipe. The test time was 720 hours, but those that were not broken at 720 hours were broken or continued until 900 hours were reached. Tables 4-1 and 4-2 show the break times of the three SSC test pieces obtained in the test, respectively. Here, in the SSC test, all three test pieces having a break time of 720 hours or more were regarded as pass.

  The chemical composition, the number of inclusions with a major axis of 5 μm or more with a composition satisfying the formulas (1) and (2), and the number of inclusions with a major axis of 5 μm or more with a composition satisfying the expressions (3) and (4) are all Inventive examples (steel pipes No. 2-1 to 2-20), which were within the scope of the invention, all had a yield strength of 862 MPa or more, and all of the three SSC tests conducted had a break time of 720 hours or more and passed.

Claims (3)

In mass%,
C: 0.25 to 0.50%,
Si: 0.01 to 0.40%,
Mn: 0.45 to 0.90%,
P: 0.010% or less,
S: 0.001% or less,
O: 0.0015% or less,
Al: 0.015 to 0.080%,
Cu: 0.02 to 0.09%,
Cr: 0.9 to 1.5%,
Mo: 1.4 to 2.0%,
Nb: 0.005 to 0.05%,
B: 0.0005 to 0.0040%,
Ca: 0.0010 to 0.0020%,
Mg: 0.001% or less,
N: containing 0.005% or less, and having a composition comprising the balance Fe and unavoidable impurities,
The organization
The number of non-metallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major axis of 5 μm or more and a composition ratio satisfying the following equations (1) and (2) is 5 or less per 100 mm ²
The number of nonmetallic inclusions in the oxide-based steel containing CaO, Al ₂ O ₃ and MgO having a major axis of 5 μm or more and a composition ratio satisfying the following equations (3) and (4) is 20 or less per 100 mm ² Yes,
Low alloy high strength seamless steel pipe for oil wells, which has a yield strength of 862 MPa or more.
(CaO) / (Al ₂ O ₃ ) ≦ 0.25 (1)
1.0 ≦ (Al ₂ O ₃ ) / (MgO) ≦ 9.0 (2)
(CaO) / (Al ₂ O ₃ ) ≧ 2.33 (3)
(CaO) / (MgO) ≧ 1.0 (4)
Here, (CaO), (Al ₂ O ₃ ) and (MgO) are mass% of CaO, Al ₂ O ₃ and MgO in non-metallic inclusions in oxide-based steel, respectively. In addition to the above composition, in mass%,
V: 0.02-0.3%,
W: 0.03 to 0.2%,
Ta: 0.03-0.3%
The low alloy high strength seamless steel pipe for oil well according to claim 1, which contains one or more selected from the following. In addition to the above composition, in mass%,
Ti: 0.003 to 0.050%,
Zr: 0.005 to 0.10%
The low-alloy high-strength seamless steel pipe for oil wells according to claim 1 or 2, which contains one or two selected from among them.